Low pressure discharge lamps are well known in the art. These lamps contain small doses of mercury which radiates under the influence of the discharge arc. In order to achieve maximum light output, it is desirable to make the discharge tube of the lamp as long as possible. In order to achieve light outputs comparable to the light output of a traditional incandescent bulb, the application of discharge tubes with a length in excess of tens of centimeters is necessary. Such tubes must be folded into various forms, in order to make them compact enough for various lamp shades and covers, which were originally designed for incandescent bulbs.
One of the popular forms of long discharge tubes is that of a double helix or double coil. This configuration is advantageous, because both ends of the discharge tube are positioned at the same ends of the coil, and a relatively long coil is easily attached to the lamp housing. The known double coil type discharge tubes normally has a constant diameter throughout the coil. However, in order to meet customer demand, it would be desirable to produce double coil type lamps with variable diameters, e. g. where the diameter of the coil is smaller towards the lamp housing. Such lamps more resemble traditional pear shaped incandescent bulbs, and hence they are found more attractive by certain customers.
A double helix type compact fluorescent lamp is disclosed in U.S. Pat. No. 5,705,883. Such a double helix configuration results in a compact discharge tube. In the lamp shown in U.S. Pat. No. 5,705,883, the double helix formed by the discharge tube has an essentially constant inner and outer diameter apart from the sealed ends of the discharge tube which are bent inwards. The bending of the tube ends is made in a separate manufacturing step. This lamp may be manufactured by winding the double coil onto a properly shaped mold, and screwing off the discharge tube from the mold. With this method, it is not possible to form a double coil where the parts away from the closed end of the coil have a much smaller diameter than the rest of the coil, because the core mold can not be screwed out. It is possible to break up the mold after the forming of the tube, but it is not feasible in mass production, due to cost and manufacturing time considerations.
Another method and apparatus for forming a coil-shaped discharge tube is disclosed in U.S. Pat. Nos. 5,766,295 and 5,824,130. In this known method, the bending sections of the discharge tube are heated to a softening temperature, and a bending moment is exerted on the bending section with the help of a first and second clamp, and a central rod. This method is only applicable for forming a single-coil shaped discharge tube.
Therefore, there is a need for a method and apparatus for forming the discharge tube of a discharge lamp which has a varying radius or direction of curvature, and particularly where the discharge tube is coil shaped with varying diameter.